Polycrystalline diamond compact drill bits are disclosed in U.S. Pat. No. 4,244,432, in which the synthetic diamond bits have their cutters arranged in many ways, including straight blades, spiral blades and uniformly distributed cutters. The bit crown, made of a hard material, such as metal bonded tungsten carbide, has profiles ranging from flat, for drilling soft to medium formations, to more steeply profiled bits for use in harder formations.
Regardless of the profile or cutter arrangement, there are common characteristics of all polycrystalline diamond compact bits. Cutting is done by a shearing action, which produces rock shavings that are considerably larger than those made by a conventional diamond bit. The fluid discharge nozzle portions of the bit are very close to the formation surface being cut and assist in the cutting action by eroding pieces of the rock beneath them. Efficient removal of the volume of cuttings produced prevents recutting of rock fragments, which reduces the stresses on the compact cutters.
It is commonly accepted that penetration rates of the bit in the formation are a function of hydraulic efficiency, as well as of the mechanical parameters, such as bit weight, rotational speed and rock strength. Because hydraulics are an integral of the drilling process, fluid mechanics must be given consideration in the design process as cutter placement density and orientation.
In order to clean the bit uniformly, polycrystalline diamond bits often have more than three nozzles. Bit size and cutter arrangement determine the number of nozzles and their orientation. The total flow area of the nozzles is determined by the hydraulic requirements found in the individual drilling situation. As the number of nozzles in the bit increases for a given total flow area, bit plugging becomes more of a problem because the orifice of the nozzle is smaller. With a round nozzle orifice, the cross-sectional area through the nozzle is relatively small, making it easier for debris to plug the nozzle orifice.